Superabrasives, such as CBN, generally cost about one hundred to one thousand times as much per unit volume as other conventional abrasives but nevertheless are cost effective in grinding certain materials such as tool steels.
CBN is used predominantly in grit sizes from about 100-400 for precision grinding processes. In general in the prior art the total grinding performance of a tool containing CBN has been found to correlate closely with the volume fraction of CBN in the tool, although a certain amount of porosity in a grinding wheel containing CBN is usually desirable. Conventional abrasives have been used together with other superabrasives in abrasive tools in the prior art, but the conventional abrasive in such products has functioned primarily as a kind of diluent and has contributed little if anything as an abrasive to the total cutting tool life when the tool was used on hard work pieces such as tool steels. For example, Makhlouf et al. U.S. Pat. 4,652,277 of Mar. 24, 1987 refers to the use of silicon carbide and alumina to help make the coefficients of thermal expansion of the rim and core regions as close to each other as possible in a grinding wheel containing CBN in the rim region only, but there is no suggestion that these conventional abrasive materials contribute directly as abrasives to the cutting performance of the wheel.
When dealing with expensive abrasives such as CBN it is a frequent expedient to form only the rim of a wheel from the expensive material. In the discussion that follows, the term "tool" will be used to denote any device designed for abrading and, in the context of grinding wheels will cover not only wheels of uniform construction but will also, for wheels of non-uniform construction, describe that peripheral portion of the wheel that contains the abrasive formulations of the invention. Thus even metal single layer., (often called MSL), wheels where the "tool" comprises a single layer of abrasive particles adhered to the rim of a wheel by a metal bond are included. In such cases however it is usual to refer to the proportions of the surface area covered by the abrasive rather than the volume of the tool represented by that abrasive component. Generally a wheel in which the maximum density of abrasive particles has been incorporated into the MSL is referred to as a 100% wheel even though the actual percentage of the surface area covered by the abrasive particles is only about 90%.